Mirrors may be used to focus high energy beams of light for various applications including the imaging process used in lithography. The power absorbed by such mirrors is expected to increase in the extreme ultraviolet (EUV) region as compared to longer wavelengths. While these mirrors can have a reflectivity that approaches 70%, the roughly 30% inefficiency leads to residual heating effects. This heating may be detrimental.
The heating of such mirrors tends to be non-uniform and may expand surfaces of the materials. These expansions may deform the material and may generate wavefront errors.
Reflectivity in the EUV spectral region is obtained through the use of multilayer interference coatings, typically alternating layers of Molybdenum and Silicon. Heating these multilayer coatings may tend to shift the center wavelength as a result of induced changes in physical thickness and index of refraction. The operation of these EUV mirrors may include narrow bandwidth reflectivity. This shift of center wavelength may reduce radiometric efficiency at the design wavelength.
All of these undesirable changes to the mirror that are based on heating may be generically referred to as aberrations. These aberrations may reduce image quality.
These heating effects can vary dependent on time, making them difficult to compensate. Feedback control of the light source, i.e., control of absolute output, may be able to compensate for the transmission changes in an optical system resulting from the temperature change of the mirrors. However, this does not overcome the loss of efficiency and may not adequately compensate for any local non-uniform heating effects that will change the projected image. The use of materials with ultra-low coefficients of thermal expansion may also improve the operation. However, the use of these materials may be limited, since they may be incompatible with the optical properties required from the multilayer coatings.